When you drill a hole in a joist or stud for NM cable, how deep does it have to be so a metal plate isn’t required?
Janet
When you drill a hole in a joist or stud for NM cable, how deep does it have to be so a metal plate isn’t required?
Janet
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Replies
Hole in Joist for Wire
I'm in Ontario, Canada and the Electrical Safety Code here says that cables must be kept back at least 32 mm (1.25 inches), from the outer surfaces of studs, joists, or similar structural members; or they must be protected from mechanical injury from driven nails, screws, or staples. Steel protection plates are most often used to protect the cable and should be made of sheet steel of at least No. 16 MSG (Manufacturers' Standard Gauge) or the equivalent. You can buy the protector plates at most big box hardware stores.
What about BX?
Just out of curiosity: Is BX considered to be already protected from driven nails, etc?
A lot of the BX in our unfinished basement goes around the outer surface of the joists. Somebody evidently thought this was insufficient, because the rest of the BX goes through holes set well back from the surface.
Janet
BX/AC needs protection
jyang949 wrote:
Just out of curiosity: Is BX considered to be already protected from driven nails, etc?
A lot of the BX in our unfinished basement goes around the outer surface of the joists. Somebody evidently thought this was insufficient, because the rest of the BX goes through holes set well back from the surface.
Janet
Nope just what is below
300.4(A)(1)
Exception No. 1: Steel plates shall not be required to protect rigid metal conduit, intermediate metal conduit, rigid nonmetallic conduit, or electrical metallic tubing.
How to drill the hole!
I'm using a Forstner bit in a cordless drill, and not making much progress. For one thing, it's hard to apply pressure when I'm reaching overhead and drilling a horizontal hole, with nothing to brace against.
Is there a special bit for drilling holes through joists? Do I need an electric drill with more oomph?
Janet
A true "Forstener" bit is a
A true "Forstener" bit is a drill-press only tool.
Milwaukee makes a similar "Selfeed" bit, which has a pilot screw and will pull itself through. Irwin's SpeedBor Max bits (available in home centers) also drills real good through solid wood.
I do not use either bit in a drill. The SpeedBor Max bits fit in any impact driver with the usual 1/4 hex collet. I have a special tool for the Milwaukee bits. An impact driver will also fit between most studs- something most drills won't do.
If you're going to use a drill, ordinary "spade" bits will work just fine. Slow, but they'll work.
A "spade bit" is the usual
A "spade bit" is the usual choice, but one needs to be a bit careful (even with the SpeedBor Max style) as spade bits tend to "grab" and can cause injury if they catch you unprepared.
I don't want to overstate this, just be careful.
Spade bit worked!
Not sure if it worked better because of its design, or because it was a smaller diameter. Probably both.
The bit got really hot, so I lubricated the bit and did the drilling in stages.
Janet
Live and learn
renosteinke wrote:
A true "Forstener" bit is a drill-press only tool.
I didn't know that. Thanks for the info.
Hole in joist
The bit renosteinke mentioned is also called an auger bit, or ship's auger. There's also less likelihood of the bit catching and rotating the drill out of your hand. You can buy short augers in the electrical department of Lowes or HD but you would need an impact driver or corded drill for an auger. I haven't had much success using them with a cordless drill.
What about more than one cable?
If you have several strands of cable to pass through joists, how do you decide how many holes to drill and how many cables share a hole?
In a joist, how close together can the holes be?
Janet
If you feed more than two cables through the same hole you may need to "derate" the cable. But the rules on this are a bit arcane.
You really only need to derate if the hole is stuffed with insulation or fire caulk.
334.80 .... Where more than two NM cables containing two or more current-carrying conductors are installed, without maintaining spacing between the cables, through the same opening in wood framing that is to be fire- or draft-stopped using thermal insulation, caulk, or sealing foam, the allowable ampacity of each conductor shall be adjusted in accordance with Table 310.15(B)(2)(a) and the provisions of 310.15(A)(2), Exception, shall not apply.
As long as you do not bundle them where they are NOT going through the holes (without insulation) you are OK, basically spread them out in the joist bay so they can breathe.
If you can prove to the inspector that they were separated where they are in the insulation, you are OK too ... Good Luck .
I suppose if you wedged a piece of styrofoam coffee cup between the cables you might have a case. Generally they want to see one hole, one cable when you are going up into the attic or other spaces that need draft stopping (spray foam).You usually don't see Romex in places that require fire stopping.
Your usual Romex cable will slip easily through a 5/8" hole. It's also a lot easier to drill a 5/8" hole than a 3/4" hole.
When I am running several cables through the same joist, I line the holes up vertically, with about 2" of wood between the holes. That way, there's no question of the holes being far enough apart.
Distance from ends of joist?
How close to the ends of the joists can holes be drilled?